A variety of control systems exist to control various parts of different processes for the separation of air by preferential adsorption. U.S. Pat. No. 4,472,177 is directed to a vacuum swing system for the recovery from air of a high purity nitrogen and an oxygen-enriched gas fraction. The system monitors the percentage of an impurity gas in the product gas as it comes out of an adsorption column and modifies the time duration of the steps of adsorption and regeneration (rinse) while keeping air input flow rate substantially constant. One problem with this type of control is that the equipment which senses the impurity gas in the product gas must have a relatively fast response time and is therefore relatively expensive. A buffer tank is usually coupled between the outputs of the chambers and a valve supplied by a customer to control the amounts of nitrogen received from the system. A second impurity measuring device is typically coupled to an inlet of a customer's valve to measure an impurity in the product gas. In addition, varying the time periods of the various steps of the process can cause disruptions in the steady state flow rate of the end gas product.
The present assignee and/or its subsidiaries market a Pressure Swing Adsorption (PSA) system which comprises two adsorption chambers. Each adsorption chamber contains a carbon structure which preferentially adsorbs oxygen from incoming air. First and second pressure regulators (mechanically adjustable pressure control valves) are in series with an inlet line to the chambers and an outlet line from the chambers, respectively. A holding tank coupled to the outlet line of the adsorption chambers collects nitrogen gas which contains oxygen as an impurity. An oxygen detector is coupled to an outlet line of the holding tank. The system is designed to automatically shut down if the oxygen content of the outlet gas exceeds a preselected limit. The amount of oxygen is manually read from the analyzer and the pressure set point of the first regulator is mechanically adjusted to control the pressure and therefore flow rate of air into the system to control the percentage of oxygen in the recovered nitrogen. After this mechanical adjustment the system is allowed to run for an hour to an hour and a half to allow it to come to a new steady state operating point. If the percentage of oxygen impurity is determined to be within preselected limits then no further adjustments are needed. If not then another adjustment of the set pressure of the first regulator is made and the system is again run until it comes to a new steady state point. This process is repeated until the oxygen impurity level is within a preselected range. If the input flow rate of air is decreased too much then the concentration of oxygen will decrease to a value below the preselected limit with output flow rate of the desired nitrogen also decreasing. The first regulator must then be readjusted to increase the input flow of air so as to increase the output flow of nitrogen while keeping the oxygen impurity percentage at a preselected level. These manual adjustments can be very time consuming and are typically required on a regular basis.
The extent of adsorbtion of oxygen by the carbon structures is a function of the ambient temperature of the incoming air and adsorption chambers. Thus the oxygen impurity level generated in the above described system is a function of the ambient temperature of the incoming air and the adsorption chambers. For example, a system can be designed to produce 4000 standard cubic feet per hour (SCFH) with 1% impurity at an ambient temperature of 70.degree. C. This system may produce an output with 1.3% impurity at 4000 SCFH at an ambient temperature of 90.degree. C. The system must therefore be designed to provide the required impurity level under worst case conditions which means at normal ambient conditions the system is over designed in terms of capacity. The need for repeated time consuming adjustments by maintenance personnel, coupled with the requirement of an over designed system results in a product in which the economies of production and operation can clearly use improvement.
It is desirable to have a PSA system in which the inlet air flow rate is automatically and continuously adjusted so as to maintain the oxygen impurity level at a preselected value while permitting the nitrogen flow rate to be as high as possible at existing temperature conditions while essentially eliminating the need for manual readjustment of the system.